Method and apparatus for an integrated laser beam scanner using a carrier substrate

ABSTRACT

An solid state scanning system having a single crystal silicon deflection mirror and scanning mirror is integrated with a light source. Separation of the micro-electro-mechanical systems and light emitters on separate substrates allows the use of flip-chip and solder bump bonding techniques for mounting of the light sources. The separate substrates are subsequently full wafer bonded together to create an integrated solid state scanning system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to “METHOD AND APPARATUS FOR ANINTEGRATED LASER BEAM SCANNER” by Floyd, Sun and Kubby (Attorney DocketNo. D/98706). Ser. No. 09/201738, filed on the same day and assigned tothe same assignee which is hereby incorporated by reference in itsentirety.

BACKGROUND AND SUMMARY OF INVENTION

The present invention relates generally to the field of laser beamscanning systems, and more particularly to micro-electro-mechanicalsystems (MEMS) for laser beam scanning. Miniature laser beam scanningsystems are important for applications such as barcode scanning, machinevision and, most importantly, xerographic printing. The use of MEMS toreplace standard raster output scanning (ROS) in xerographic printengines allows simplification of printing systems by eliminatingmacroscopic mechanical components and replacing them with large arraysof scanning elements. Advanced computation and control algorithms areused in managing the large arrays of scanning elements. Such MEMS basedprinting systems are entirely solid state, reducing complexity, andallowing increased functionality, including compensation of errors orfailures in the scanner elements.

An important step in constructing solid state scanning systems isintegration of the semiconductor light emitter directly with MEMSactuators to gain the desired optical system simplification. Integratedscanners, which have lasers and scanning mirrors in the same structure,have been demonstrated using manual placement of laser chips onto MEMSwafers with micromachined alignment parts and adhesives by L. Y. Lin etal in Applied Physics Letters, 66, p. 2946, 1995 and by M. J. Daneman etal in Photonics Technology Letters, 8(3), p. 396, 1996. However, currenttechniques do not allow for wafer-scale integration of the light-emitterand MEMS device.

In accordance with the present invention a laser beam scanner consistingof a single crystal silicon deflection mirror and a torsional mirror isintegrated with a laser diode in the same structure. Details of creatinga torsional mirror and actuating it magnetically or electrostaticallyare detailed in U.S. Pat. No. 5,629,790 by Neukermans and Slater whichis incorporated herein by reference in its entirety.

Using solder bump bonding methods, completed and tested laser diodes arebonded to a glass or a silicon carrier substrate. The carrier substrateis aligned and bonded to a Si or SOI wafer containing the MEMS layers.Bonding of the lasers to a carrier substrate completely partitions thebonding process from the MEMS. This complete partition eliminatespossible conflicts between the conditions needed for solder bumpbonding, such as the use of solder flux, and preserves the integrity ofthe MEMS layers.

The substrates are heated in a non-oxidizing environment to join the twosubstrates. High surface tension of the solder aligns the wettable metalbonding pads on each substrate with each other. The ability of thereflowed solder to self-align the substrates because of surface tensionsimplifies assembly.

The use of the SCS layer of a SOI wafer, rather than a polysilicon filmprovides for the introduction of very flat and smooth mirrors and highreliability torsion bars. The device is scalable to arrays of lasers andscanning mirrors.

Integration of the scanner and light source eliminates the need forexternal, manual alignment of light sources and scanning mirrors.Simplified post-processing steps such as interconnect metallization canbe realized because the use of an etched recess results in nearly planarsurfaces. In addition, pick and place technologies used for multi-chipmodule assembly can be adapted for wafer scale assembly and bonding oflight sources to the carrier substrate.

Thus, the present invention allows the integration of lasers, electricalinterconnects, and electrodes on a single glass or Si wafer foractuation of MEMS devices. The glass or Si wafer is aligned and bondedto the MEMS wafer, forming an integrated, three dimensional structure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained and understood by referringto the following detailed description and the accompanying drawings inwhich like reference numerals denote like elements as between thevarious drawings. The drawings, briefly described below, are not toscale.

FIG. 1 is shows MEMS layers and VCSEL for a laser beam scanner inaccordance with an embodiment of this invention.

FIG. 2a shows a laser beam scanner with optical path having anelectrostatically actuated torsion mirror in accordance with anembodiment of this invention.

FIG. 2b shows a top view a laser beam scanner in accordance with anembodiment of this invention.

FIG. 3a shows a laser beam scanner with optical path having amagnetically actuated torsion mirror in accordance with an embodiment ofthis invention.

FIG. 3b shows a laser beam scanner with optical path having amagnetically actuated torsion mirror using an external magnetic field inaccordance with an embodiment of this invention.

FIGS. 4a-4j show process steps for fabricating MEMS components inaccordance with an embodiment of this invention.

FIGS. 5a-5e show steps for fabricating substrate containing laser dieand mirror actuation electrodes in accordance with an embodiment of thisinvention

FIG. 6a shows a completed laser beam scanner before release ofdeflecting mirror in accordance with an embodiment of this invention.

FIG. 6b shows a completed integrated solid state scanner after releaseof deflecting mirror in accordance with an embodiment of this invention.

DETAILED DESCRIPTION

An embodiment in accordance with the present invention is shown in FIG.1 and FIG. 2a. A laser beam scanner consisting of single crystal silicon(SCS) deflecting mirror 240 and torsional mirror 250 is integrated withlaser diode or light emitting diode 105. Using solder bump bondingmethods, completed and tested laser diodes 105 are bonded to glass orsilicon carrier substrate 101. Carrier substrate 101 is aligned andbonded to MEMS substrate 130 containing the MEMS layers. Bonding oflaser diode 105 to carrier substrate 101 completely partitions thebonding process from the MEMS layers. This complete partition eliminatespossible conflicts between the conditions needed for solder bumpbonding, such as the use of solder flux, and preserves the integrity ofthe MEMS layers. Typically, solders such as Pb/Sn, Au/Sn, or In/Sn areevaporated selectively onto wettable metal bonding pads 111 ontosubstrate 101 and reflowed to form hemispherical solder bumps 110.Solder bumps 110 are contacted to wettable metal bonding pads 113 onlaser substrate 106.

Laser substrate 106 and carrier substrate 101 are heated in anon-oxidizing environment to join the respective substrates together.High surface tension of the solder aligns wettable metal bonding pads 111 with wettable metal bonding pads 113 on laser substrate 106. Theability of the reflowed solder to self-align laser substrate 106 withcarrier substrate 101 because of surface tension simplifies the assemblyprocess. Additionally, very little pressure is required during theprocess of bonding laser substrate 106 to carrier substrate 101.

Micromechanical elements (MEMS) are formed on MEMS substrate 130,typically about 500 μm thick using conventional photolithography and thepatterning of single crystal silicon (SCS) layer 118, polysilicon layers117 and insulating oxide layers 116, which are typically PSG or thermaloxide, is performed using both dry and wet etching techniques. MEMSsubstrate 130 embodies SCS layer 118, insulating oxide layer 116 andsilicon substrate 115. Typical thickness for each of layers 116, 117,and 118 is on the order of several μm. VCSEL (vertical cavity surfaceemitting laser) 105 is solder bump 110 bonded to glass ordielectric-coated (typically SiO₂ or Si₃N₄ coated) Si substrate 101,typically about 500 μm thick. Additionally, two actuation electrodes 220and two interconnects 125 are formed on glass or dielectric-coated Sisubstrate 101. Interconnects 125 provide current to substrate 101 topower VCSEL 105 and to electrodes 220 for control of torsional mirror250. After solder bonding of VCSEL 105 to glass or dielectric-coated Sisubstrate 101, substrate 101 is aligned and bonded to MEMS substrate130.

MEMS substrate 130 has deep reactive ion etching (RIE) and/or wet etchedhole 135, typically 3 mm in diameter, for emitted light 299 (see FIG.2a) to pass through MEMS substrate 130 and onto deflecting mirror 240.Deflecting mirror 240 reflects emitted light 299 onto torsional mirror250. As shown in FIG. 2a, polysilicon hinge 255 attaches deflectingmirror 240 to MEMS substrate 130. Deflecting mirror 240 is etched fromSCS layer 118. Polysilicon hinge 255 allows deflecting mirror 240 torotate clockwise about an axis perpendicular to the plane of FIG. 2a,out of MEMS substrate 130 to the position above via 135 as shown in FIG.2a. Deflecting mirror 240 is supported by support latch 268 controlledby a spring and latch assembly (not shown) in the manner described inthe paper by Lin et al. in Photonics Technology Letters, 6(12), p. 1445,1994 which is incorporated herein in its entirety by reference.Controlling the position and length of support latch 268 allows theangle of deflecting mirror 240 to be precisely fixed. Deflection oftorsional mirror 250 in both directions is accomplished by chargingalternately one of two actuator electrodes 220. Torsional mirror 250 iselectrically grounded and attracted to charged one of two actuatorelectrodes 220.

FIG. 2b shows a top view of one combination deflection mirror/torsionalmirror solid state element. Polysilicon hinges 255 and deflecting mirror240 are shown along with hole 265 to receive the tab (not shown) onsupport latch 268. The layout of torsional mirror 250 supported bytorsion bar 270 with respect to hole 217 is also shown.

MEMS components such as deflecting mirror 240 and torsional mirror 250can be formed in MEMS substrate 130 by using a combination of well-knownsurface and bulk micro-machining techniques. Polysilicon hinges 255 maybe formed as described by M. C. Wu, “Micromachining for Optical andOptoelectronic Systems,” Proceedings of IEEE, Vol. 85, p. 1833, 1997 andby Pister et al., “Microfabricated Hinges,” Sensors and Actuators, A:Physical v. 33 n. 3 pp. 249-256, June 1992 which are hereby incorporatedby reference in their entirety.

As seen in FIG. 1, bonding of VCSEL 105 to glass or SiO₂ coated Sisubstrate 101 completely separates the bonding process from the MEMScomponents. The separation eliminates possible conflicts betweenconditions needed for solder bump bonding, such as the use of solderflux and the integrity of the MEMS layers. Full wafer bonding of glassor dielectric-coated Si substrate 101 to MEMS substrate 130 is done atlow temperature to avoid damage to VCSEL 105. Metallization on glass ordielectric-coated Si substrate 101 is achieved by use of adhesivebonding techniques requiring temperatures of between 20° C.-100° C.

Another embodiment in accordance with the present invention is shown inFIG. 3A. VCSEL (vertical cavity surface emitting laser) 105 is solderbump 110 bonded to glass or dielectric-coated Si substrate 101. Glass ordielectric-coated Si substrate 101 is aligned and bonded to MEMSsubstrate 130. MEMS substrate 130 has deep RIE and/or wet etched via 135for emitted light 199 to pass through the surface of MEMS substrate 130and onto deflecting mirror 240 which reflects emitted light 299 ontotorsional mirror 250. Torsional mirror 250 contains ferro-magnetic thinfilm 330 with magnetization in the plane of torsional mirror 250. Coil380 on glass or dielectric-coated Si substrate 101 generates magneticfield 391 perpendicular to the magnetic field created by ferromagneticthin film 330 contained on torsional mirror 250. Hence, actuation ofcoil 380 turns torsional mirror 250. Polysilicon hinge 255 attachesdeflecting mirror 240 to MEMS substrate 130. Polysilicon hinge 255allows deflecting mirror 240 to rotate clockwise about an axisperpendicular to the plane of FIG. 3a, out of MEMS substrate 130 to aposition above via 135 as shown in FIG. 3a. Deflecting mirror 240 can besupported by support latch 268 controlled by a spring and latch assembly(not shown) in the manner shown by Lin et al. in Photonics TechnologyLetters, 6(12), p. 1445, 1994 and incorporated herein in its entirety byreference. Fixing the position and length of support latch 268 allowsthe angle of deflecting mirror 240 to be precisely fixed.

FIG. 3b shows an embodiment in accordance with this invention whereintorsional mirror 250 contains microfabricated coil 350 generatingmagnetic field 385 perpendicular to torsional mirror 250 but isotherwise similar to FIG. 3a. Coil 350 is a conductive loop which may beformed by vapor depositing conductive material onto torsional mirror 250and patterning into coil 350. External magnetic field 370 is appliedparallel to the plane of torsional mirror 250 to turn torsional mirror250. Application of current to coil 350 results in an angular deflectionof torsional mirror 250 proportional to the current introduced into coil350. Hence, coil 350 behaves like a galvanometer coil. Direction ofcurrent flow in coil 350 determines the direction of the angulardeflection of torsional mirror 250.

Steps for fabricating deflecting mirror, supporting latch and VCSEL inaccordance with this invention are shown in FIGS. 4a-4j and FIGS. 5a-5e.The starting material is MEMS substrate 130 which comprises a silicon oninsulator material (SOI). MEMS substrate 130 includes silicon substrate115, thermally-grown SiO₂ layer 116 bonded to wafer 113. MEMS substrate130 is then thinned to the required thickness. MEMS substrates 130 arecommercially available from, for example, Bondtronix, Inc. of Alamo,Calif. or Ibis Technology Corporation of Danvers, Mass. Typicalthickness of SCS layer 118 is 2-20 μm depending on the requiredstiffness of the torsional spring elements and mirror surfaces to beconstructed. Other MEMS layers are deposited on top of MEMS substrate130 by well-known methods such as low pressure chemical vapor deposition(LPCVD). These MEMS layers include mechanical layers of polycrystallinesilicon (polysilicon) 117 (not shown in FIGS. 4) and sacrificial oxidelayer 119 that is phosphorus-doped glass (PSG). The embodiment in FIG.4a 4c has PSG layer 119 deposited directly on top of SCS layer 114 118.Polysilicon layer 117 (see FIG. 1) is subsequently deposited on PSGlayer 119. Typical thicknesses for polysilicon layer 117 and PSG layer119 are 1-2 μm.

Formation of MEMS elements occurs by conventional photolithography andpatterning of SCS layer 114 118, polysilicon layer 117, and PSG layer119 is performed using both wet and dry etching. In accordance with anembodiment of this invention, deflecting mirror 240 and deep recess 135are required.

FIGS. 4a-4j show steps for fabricating deflecting mirror 240, torsionalmirror 250, supporting latch 268, and deep recesses 135 and 217. Latch268 has a tab (not shown) which inserts into corresponding hole 165 inthe bottom of deflecting mirror 240. The final configuration ofdeflecting mirror 240 and latch 255 are shown in FIG. 6b. Typical sizesfor deflecting mirror 240 are between 0.5 mm² to 1 mm².

FIG. 4a has silicon nitride (SiN_(x)) deposited on substrate 130 usingLPCVD. SiN_(x) layer (not shown) is patterned using CF₄/O₂ RIE with aphotoresist mask. Potassium hydroxide (KOH) is used to etch holes fromthe bottom of substrate 115, stopping on layer 116. Size of hole 217 issimilar to torsional mirror 250 to allow free rotation. Hole 135 issimultaneously etched, for fitting VCSEL 105 which typically hasdimensions of 500 μm by 500 μm. Alternatively, holes 217 and 135 may bedefined by deep RIE using C₄F₈ and SF₆ with a mask of SiN_(x) orphotoresist.

FIG. 4b shows recess 135 (200-250 μm deep) etched into MEMS substrate130 using a combination of CF₄/O₂ RIE for etching SCS layer 114 118andinsulator layer 116 and a deep RIE of recess 135 using C₄F₈ and SF₆.

FIG. 4c shows CVD deposition of PSG layer 119.

FIG. 4d shows the wet etch of windows 410 into PSG layer 119 down to SCSlayer 114 118.

FIG. 4e shows deposition of aluminum film 430 (typically 0.1-0.2 μmthick) as a high reflectivity layer.

FIG. 4f shows a wet etch (typically a mixture of phosphoric and nitricacid) of aluminum film 430 to remove aluminum in all but the mirrorregions. The mirror region locations coincide with the locations ofwindows 410.

FIG. 4g shows the etch of vias 433 using CF₄/O₂ RIE with a photoresistmask. This step also serves to open laser die window 135.

FIG. 4h shows formation of hinges 255 for deflecting mirror 240 frompolysilicon layer 117 (not shown, see FIG. 1) that is deposited in thisstep.

FIG. 4i shows etch of PSG layer 119 and SCS layer 114 118to patterndeflecting mirror 240, hinges 255 and access holes 437. A typical sizefor access holes 437 is 10 μm by 10 μm. Access holes 437 allow for theetchant used to release deflecting mirror 240 to reach insulating layer116. Deflecting mirror 240 size is typically from 1 mm²-2 mm². Torsionalmirror 250 is also defined in this step.

FIG. 4j shows release of deflecting mirror 240, torsional mirror 250 andhinge 255 by etching PSG layer 119 and layer 116 using an HF based etch.

FIGS. 5a-e show the steps used to fabricate wafer 103 containing VCSEL105 and mirror actuation electrodes 220 in accordance with an embodimentof this invention.

FIG. 5a shows starting glass or silicon substrate 101 for fabrication ofwafer 103.

FIG. 5b shows deposition of silicon nitride or silicon dioxide layer 502by LPCVD or plasma-enhanced CVD process to provide electrical isolationfrom silicon substrate 101.

FIG. 5c shows deposition of electrodes 220 and solder for solder bumps110.

FIG. 5d shows completed deposition of electrodes 220 for mirroractuation. Electrodes 220 are much thicker ˜200-300 μm) than solderbumps 110 (typically 50-100 μm) and are electroplated.

FIG. 5e shows alignment and solder bump bonding of VCSEL 105 to Sisubstrate 101 in the GaAs bonding step. Solder bumps 110 can be definedon metal bonding pads 113 of VCSEL substrate 106. Si substrate 101 andVCSEL substrate 106 are heated to allow solder to flow and contactwettable metal bonding pads 111 on Si substrate 101.

FIG. 6a shows integration of substrate 101 with MEMS substrate 130 usingwell-known procedures of adhesive bonding while FIG. 6b shows thefinished assembly with raised deflecting mirror 240 locked into placewith latch 168.

Linear arrays of lasers can be bonded in a similar way; the extent ofthe array being perpendicular to the cross section shown in FIG. 6a.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications, and variations that fallwithin the spirit and scope of the appended claims.

What is claimed is:
 1. An integrated laser beam scanning structurecomprising: a first wafer having a first surface and a second surface,said wafer having a recess piercing said first surface and said secondsurface; a layer having a first region and a second region, said layerbeing attached to said first surface; a deflecting mirror fashioned fromsaid first region of said layer; a torsional mirror fashioned from saidsecond region of said layer; a second wafer having a first side; and alight source mounted on said first side of said second wafer, said firstside of said second wafer being attached to said second surface of saidfirst wafer such that said light source occupies said recess whereby alight beam emitted from said light source is deflected by saiddeflecting mirror onto said torsional mirror.
 2. The structure of claim1 wherein said first wafer is a silicon on oxide wafer.
 3. The structureof claim 1 wherein said layer is a single crystal silicon layer.
 4. Thestructure of claim 1 wherein said light source is a semiconductor lightemitter.
 5. The structure of claim 4 wherein said semiconductor lightemitter is mounted on said first side of said second wafer using solderbumps.
 6. The structure of claim 4 wherein said semiconductor lightemitter is a VCSEL chip.
 7. The structure of claim 1 wherein said recessis deep reactive ion etched.
 8. The structure of claim 1 wherein saidtorsional mirror is actuated by a pair of electrodes.
 9. The structureof claim 1 wherein said torsional mirror is actuated by a thin filmcoil.
 10. The structure of claim 1 wherein a ferromagnetic thin filmcoil is attached to said torsional mirror.
 11. The structure of claim 1wherein a thin film coil is attached to said torsional mirror.
 12. Amethod for making an integrated laser beam scanner comprising the stepsof: providing a first wafer having a first surface and a second surface,said wafer having a recess piercing said first surface and said secondsurface; attaching a layer having a first region and a second region tosaid first surface of said first wafer; fashioning a deflecting mirrorfrom said first region of said layer; fashioning a torsional mirror fromsaid second region of said layer; providing a second wafer having afirst side, said second wafer having a light source mounted on saidfirst side; and attaching said first side of said second wafer to saidsecond surface of said first wafer such that said light source occupiessaid recess whereby a light beam emitted from said light source isdeflected by said deflecting mirror onto said torsional mirror.
 13. Themethod of claim 12 wherein said layer is a single crystalline siliconlayer.
 14. The method of claim 12 wherein said light source is asemiconductor light emitter.
 15. The method of claim 14 wherein saidsemiconductor light emitter is a VCSEL chip.
 16. The method of claim 14wherein said semiconductor light emitter is mounted using solder bumps.17. The method of claim 12 wherein said torsional mirror is actuated bya pair of electrodes.
 18. The method of claim 12 wherein said torsionalmirror is actuated by a thin film coil and an external magnetic field.19. The method of claim 12 wherein a ferromagnetic thin film coil isattached to said torsional mirror.
 20. The method of claim 12 wherein athin film coil is attached to said torsional mirror.
 21. A MEMSformation method including: providing a single crystal silicon layer;forming at least one first MEMS component by patterning the singlecrystal silicon layer; depositing at least one layer of polysilicon onthe patterned single crystal silicon; and forming at least one secondMEMS component by patterning the polysilicon.
 22. The method of claim 21wherein the single crystal silicon layer is bonded to an insulator layerin a SOI wafer and providing a single crystal silicon layer comprisesproviding a SOI wafer.
 23. The method of claim 21 wherein the at leastone second MEMS component is a hinge.
 24. The method of claim 23 whereinthe at least one first MEMS component is a mirror retained by the hinge.25. The method of claim 21 wherein depositing at least one layer ofpolysilicon includes chemical vapor deposition.
 26. The method of claim21 wherein forming at least one first MEMS component includes forming adeflecting mirror.
 27. A MEMS formation method including: providing asingle crystal silicon layer; forming at least one first MEMS componentby patterning the single crystal silicon layer; depositing at least onelayer of polysilicon on the patterned single crystal silicon; andforming at least one second MEMS component by patterning thepolysilicon, the at least one second MEMS component including a hingeretaining a deflecting mirror.
 28. The method of claim 27 whereinforming at least one first MEMS component further includes forming atorsional mirror, and the method further comprises forming a recess inthe single crystal silicon layer and directing a light beam through therecess at the deflecting mirror so that the deflecting mirror deflectslight to the torsional mirror.
 29. A MEMS device comprising: at leastone single crystal silicon component; and a hinge derived from a layerof polysilicon applied over the at least one single crystal siliconcomponent.
 30. The MEMS device of claim 29 wherein the at least onesingle crystal silicon component is bonded to an insulator that rests ona handle wafer as a result of being formed from a single crystal siliconlayer of a SOI wafer.
 31. The MEMS device of claim 29 wherein the atleast one single crystal silicon component comprises a deflectingmirror.
 32. The MEMS device of claim 31 wherein the hinge retains thedeflecting mirror.
 33. The MEMS device of claim 29 wherein the at leastone single crystal silicon component comprises a torsional mirror.
 34. AMEMS device comprising: at least one single crystal silicon component;at least one polysilicon component derived from a layer of polysiliconapplied over the at least one single crystalline silicon component; anda semiconductor light emitter mounted on a substrate bonded to asupporting structure of the at least one single crystal siliconcomponent and oriented to emit a light beam at the at least one singlecrystal silicon component.
 35. The MEMS device of claim 34 wherein theat least one single crystal silicon component is bonded to an insulatoras a result of having been formed from a single crystal silicon layer ofan SOI wafer to which the semiconductor light emitter substrate isbonded.
 36. The MEMS device of claim 35 wherein the SOI wafer includes arecess into which the semiconductor light emitter projects.
 37. The MEMSdevice of claim 34 wherein the at least one single crystal siliconcomponent comprises a deflecting mirror at which the light beam isdirected and a torsional mirror to which the deflecting mirror deflectsthe light beam, and the at least one polysilicon component comprises ahinge retaining the deflecting mirror.